Multiply the following complex numbers, marked as blue dots on the graph: $[4(\cos(\frac{5}{6}\pi) + i \sin(\frac{5}{6}\pi))] \cdot [2(\cos(\frac{17}{12}\pi) + i \sin(\frac{17}{12}\pi))]$ (Your current answer will be plotted in orange.)
Solution: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $4(\cos(\frac{5}{6}\pi) + i \sin(\frac{5}{6}\pi))$ ) has angle $\frac{5}{6}\pi$ and radius $4$ The second number ( $2(\cos(\frac{17}{12}\pi) + i \sin(\frac{17}{12}\pi))$ ) has angle $\frac{17}{12}\pi$ and radius $2$ The radius of the result will be $4 \cdot 2$ , which is $8$ The sum of the angles is $\frac{5}{6}\pi + \frac{17}{12}\pi = \frac{9}{4}\pi$ The angle $\frac{9}{4}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{9}{4}\pi - 2 \pi = \frac{1}{4}\pi$ The radius of the result is $8$ and the angle of the result is $\frac{1}{4}\pi$.